Methods, Materials and Techniques for Precious Metal Recovery

ABSTRACT

Materials and methods for precious metal recovery are disclosed. Usable leaching solutions are preferably aqueous based and include appropriate materials in sufficient quantities to solubilize and stabilize precious metal. Such materials typically include phosphoric acid and an oxidant material. Some or all of the oxidant material can be, in some instances, generated in-situ. The leaching solution is typically contacted with a substrate having a target precious metal, thereby solubilizing precious metal to form a stable, pregnant solution. The precious metal can then be recovered from the pregnant solution. In some instances, components of the leaching solution can be regenerated and reused in subsequent leaching.

REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser. No. 62/624,842, filed on Feb. 1, 2018, the disclosure of which is incorporated herein by reference in its entirety.

FIELD OF DISCLOSURE

The present disclosure relates to methods, materials and techniques for precious metal recovery. Example applications relate to the preparation of leaching solutions and use of resulting leaching solutions in the recovery of precious metals from substrates, such as gold bearing ores, concentrates, anodic slimes and residues, electronic waste, metallic scrap, and materials previously treated by roasting, bacterial leaching, pressure leaching, or other techniques used to liberate precious metal from its matrix. Many of the materials, methods, and techniques disclosed herein are particularly advantageous for gold recovery.

BACKGROUND

Precious metal in substrate such as mined ore can be recovered by contacting the substrate with leaching solution. Typically, the leaching solution is an aqueous-based solution. Generally, the leaching solution is contacted with substrate to solubilize the precious metal. Thereafter, valuable components of the solution, such as the solubilized precious metal, are recovered.

Issues with past approaches relate to: efficiency of leaching recovery; avoidance of undesirable materials such as cyanide materials; and generation of stable solutions. Improvements in at least one, and sometimes more, of these issues are sought.

SUMMARY

Techniques and materials disclosed herein relate to extracting precious metal from substrate into solution, for example, using leaching solutions. In addition, techniques and materials disclosed herein relate to methods for recovering precious metal from the leaching solutions. Techniques and materials disclosed herein also relate to methods for regenerating components of leaching solutions.

There is no specific requirement that a material, technique or method relating to a leaching solution include all of the details characterized herein, in order to obtain some benefit according to the present disclosure. Thus, the specific examples characterized are meant to be exemplary applications of the techniques described, and alternatives are possible.

According to certain techniques, an aqueous-based leaching solution for a precious metal can include an iodide salt, a chlorite salt and/or an iodate salt, phosphoric acid, and optionally a chloride and/or bromide salt in an amount effective to enhance leaching. The chlorite salt can be used independently from, or in combination with, the iodate salt. Preferably, the leaching solution has a pH of no greater than 7, and is characterized by an oxidation reduction potential (ORP) in a range from 540 mV to 650 mV.

In various applications, a stable, aqueous-based, precious metal-containing leachate results from leaching precious metal oxidizable with an aqueous-based leach solution. Various leaching methods are contemplated, such as continuous or batch stirred tank agitation, vat leaching, or in situ techniques such as dump and heap leaching.

Generally, a method of recovering a precious metal from a precious metal-containing pregnant leach solution includes recovering the precious metal by various methods, such as electrowinning, precipitation, cementation, ion exchange, and/or adsorption onto activated carbon, thereby generating a barren solution and passing current through the barren solution having a triiodide precursor to increase a triiodide content of the barren solution.

A representative method for recovering a precious metal from a precious metal-containing substrate can comprise (i) combining the precious metal-containing substrate with any aqueous-based leaching solution disclosed herein to form solids and a pregnant leach solution, and (ii) separating the solids from the pregnant leach solution. In some embodiments, this method can further comprise (iii) removing the precious metal from the pregnant leach solution, thereby forming a barren leaching solution, and (iv) increasing the oxidation reduction potential (ORP) of the barren leaching solution to at least 540 mV to form a regenerated aqueous-based leaching solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow diagram indicating steps in a process for precious metal recovery.

FIG. 2 is a schematic flow diagram in accordance with FIG. 1 indicating example steps in a recovery of gold from a substrate.

FIG. 3 is a graph showing experimental results described herein below.

FIG. 4 is a graph showing experimental results, including data of FIG. 3, described herein below.

FIG. 5 is a graph showing experimental results described herein below.

FIG. 6 is a graph showing experimental results described herein below.

FIG. 7 is a graph showing experimental results described herein below.

FIG. 8 is a graph showing experimental results described herein below.

FIG. 9 is a schematic flow diagram in accordance with FIG. 1 indicating example steps in a recovery of gold from a substrate

DETAILED DESCRIPTION I. Issues, Materials, and Techniques Related to Precious Metals Recovery Generally

A. General

Precious metal recovery involves separation of the precious metal, such as gold, from ore, mining tails, and/or waste such as electronic waste. The techniques disclosed and contemplated herein include treating substrate with aqueous-based leach solution to produce a “pregnant solution.” Herein, the term “pregnant solution” is meant to refer to a leach solution having leached precious metal therein.

Also described herein are processes used to recover the precious metal from the pregnant solution and processes relating to regeneration of various components of the aqueous based leach solution. The techniques disclosed and contemplated by this disclosure are particularly well-adapted for processes that involve the recovery of gold as the precious metal.

B. Leaching Solutions

Aqueous-based leaching solutions usable in techniques characterized herein are those that, when in contact with substrate, solubilize at least a portion of precious metal in the substrate by oxidizing the precious metal. Typical leaching solutions characterized herein are effective under a wide pH range, such as between a pH of 3 to 10. Typically, the aqueous-based solutions characterized herein have at or near-neutral pH, for instance, in the range of pH 4-7. Usually, the aqueous-based solutions characterized herein have a pH of at least 4. Preferably, the aqueous-based leaching solutions characterized herein are provided with a pH within the range 5-7, inclusive. As used herein, “inclusive”, in this context, means the end points are included within the range.

Typically, leaching solutions characterized herein preferably are practiced in a manner that avoid the use of soluble, inorganic cyanide materials (such as cyanide salts). Cyanide materials are preferably avoided in the leaching solutions characterized herein because, at least, they are hazardous and subject to stringent environmental regulations.

Typically, leaching solutions characterized herein include iodate material, iodide material, and one or more weak acids. In some implementations, chlorite partially or completely replaces the use of iodate material. During typical leaching operations, electrochemical cell(s) are used to maintain the oxidation reduction potential of leaching solutions. Typically, acid is added to a barren solution (the solution resulting from removing precious metal from a leaching solution) to partially regenerate the barren solution before or after using electrochemical cell(s) to fully regenerate the leaching solution for further use. Typically, divided electrochemical cells are used to regenerate the leaching solution. In some embodiments, electrochemical cell(s) used for regeneration utilize conducting diamond electrodes. Electrochemical cells are discussed in greater detail below.

It is noted that in mining applications, the leaching solution is sometimes referred to as a lixiviant. Herein, when the term “leaching solution” is used, it is meant to include lixiviant(s).

1. Iodine Material

The instantly-disclosed aqueous-based leaching solutions include iodine material, which includes iodide material and iodate material. Iodide material includes compounds capable of forming iodide in an aqueous solution, such as triiodide. Iodide material includes iodide salt, such as KI.

Preferably, pure elemental iodine is not added to the leaching solution. Rather, without being bound by a particular theory, it is speculated that triiodide material is formed electrochemically. Without being bound by a particular theory, it is speculated that iodide material is useful for complexing and solubility of precious metal.

The instantly-disclosed aqueous-based leaching solutions can also include iodate material. Iodate material includes compounds capable of forming iodate in an aqueous solution. Example iodate material includes iodate salt, such as potassium iodate (KIO₃).

Some implementations replace the use of some or all iodate material with chlorite material. Example chlorite material includes chlorite salt, such as sodium chlorite and/or potassium chlorite. Generally, chlorite salt is cheaper to obtain than iodate salt. Thus, using chlorite salt in place of some or all of the iodate salt can reduce costs.

2. Acidic Components

As indicated above, the leaching solutions disclosed herein include acidic material. The acidity is preferably provided by weak acid material. Weak acid material can facilitate leaching of the target metal. Additionally, without being bound by a particular theory, it is speculated that weak acid material improves the ability of the leaching solution to hold the precious metal in solution. Examples of weak acid material include, but are not limited to, carboxylic acids and phosphoric acid. As used herein, weak acid material includes the acid, a salt of the acid, or a combination thereof.

One example of water-soluble, carboxylic acid material is citric acid. Citric acid is a weak, organic acid. Citric acid has pKa values of 3.13, 4.76, and 6.4. Another example is acetic acid.

Without being bound by a particular theory, it is speculated that citric acid improves the ability of the instant leaching solutions to solubilize precious metal. Although citric acid is preferable, acetic acid can be used in place of citric acid.

In some instances, the leaching solutions can include both citric acid and acetic acid material. Acetic acid material includes glacial acetic acid, dry acetic acid, or equivalents thereof.

Another example of an acidic component is boric acid. Boric acid is a weak, inorganic acid. Boric acid has pKa values of 9.24, 12.4 and 13.3. Without being bound by a particular theory, it is speculated that boric acid improves the ability of the instant leaching solutions to solubilize precious metal. Without being bound by a particular theory, it is speculated that boric acid acts as a buffer in the instant leaching solutions. Optionally, borates can be used in place of boric acid. Storage stability is discussed in more detail below.

Another example of acidic material is phosphoric acid. Phosphoric acid is a polyprotic, weak, inorganic acid. Phosphoric acid has pKa values of 2.12, 7.20, and 12.3. It has been observed that the use of phosphoric acid is preferable to the use of carboxylic acids. Without being bound a particular theory, the use of carboxylic acid appears to create a condition whereby base metal concentrations in the leaching solution increase over time. That is, as the leaching solution is used and regenerated, the concentration of base metals in solution increases. Base metal in solution can hinder precious metal recovery, for example, from yield, purity, and/or energy expended perspectives.

3. Bromine or Chlorine Material

The instantly-disclosed aqueous-based leaching solutions also optionally include bromine material or chlorine material (“halogen material”). Halogen material is usually added as a sodium salt or a potassium salt. Typical halogen material is chloride salt or bromide salt. Accordingly, example halogen material can include bromide salt material, such as potassium bromide and/or sodium bromide, and chloride salt material, such as potassium chloride and/or sodium chloride.

4. Oxidants

The leaching solutions disclosed herein preferably include triiodide material and iodide material for dissolution of the precious metal during leaching. Triiodide material is typically generated through oxidation of iodide electrochemically on electrochemical cell anode and/or optionally by addition of a chemical oxidant such as hypochlorite, hydrogen peroxide, persulfate (e.g., potassium monopersulfate, potassium persulfate, and sodium persulfate), ozone, or other materials capable of oxidizing iodide to iodine. Alternatively, triiodide material can be generated using iodate material with addition of acid materials (i.e., mixing iodate material with an acid). Alternatively, triiodide material can be generated using chlorite salt material.

Preferably, triiodide material is generated in-situ through electrochemical approaches from relatively inexpensive, safe and easy to use sources, such as iodide materials, iodate materials, and/or chlorite materials. Examples of triiodide material production using a chemical oxidant are provided below. An example pathway using hydrogen peroxide starting material is as follows:

H₂O₂+2I⁻+2H⁺→I₂+2H₂O  (1)

The reaction in equation (1) is followed by:

I₂+I⁻→I₃ ⁻  (2)

Another example pathway is using persulfate.

S₂O₈ ²⁻+2I⁻→I₂+2SO₄ ²⁻  (3)

The reaction in equation (3) is followed by reaction of iodine with iodide to form triiodide ion:

I₂+I⁻→I₃ ⁻  (4)

5. Cyanide Avoidance

Many precious metal recovery operations use cyanide-based solutions for the leachate. Generally, in contrast, cyanides and cyanide salts are avoided or limited in the instantly-disclosed leaching solutions. Because the instant leaching solutions preferably have no cyanide added, the disclosed solutions avoid the environmental impacts, hazards, and regulations attendant to the use of cyanide.

Preferably, the leaching solutions disclosed herein have no more than 0.0001 wt % cyanide (e.g. NaCN) added. As used herein, “wt %” means the weight percent of all ingredients, including the solvent, combined to form the leaching solution. Most preferably, the leaching solutions disclosed herein have no cyanide (e.g. NaCN) added. However, the disclosed leaching solutions can acquire some cyanide during the leaching process.

6. Passivating Agents

Leaching solutions disclosed herein preferably include one or more passivating agents. Generally, the use of a passivating agent results in a surface coating or layer on a material that prevents or protects the material from reaction, such as with water. For example, passivated metals include a surface coating, such as a metal oxide, that prevents/protects the metal from reaction.

In the instant implementations, materials desirable for passivating include various soluble species that might dissolve from mineral material surfaces and/or metal material surfaces. Soluble species include dissociated ions. Example minerals include sulfide minerals, such as pyrite (FeS₂). Example metal material includes base metal- and alloy-containing materials. Example base metals include iron, zinc, nickel, copper, and tin. An example alloy is solder. Passivating agents can be used with all substrate materials contemplated herein, including e-waste, ores, and concentrates.

Without being bound by a particular theory, it is speculated that, without the passivating agent(s), materials, such as sulfides, can leach into the leaching solution and precipitate on the surface of gold. The precipitate was speculated to create a non- or semi-permeable layer that undesirably prevented or limited the leach solution from attacking the gold and dissolving the gold into solution.

It is also speculated that adding the passivating agent results in a coating on the surface of non-target substances, such as pyrite or copper. The coating prevents leaching or excessive leaching of the non-target substances into the solution. Additionally, by not leaching non-target substances via the use of passivating agents, the chemical demand on iodide species is reduced. In other words, iodide in the leaching solution is not reacting with, or substantially less iodide is reacting with, non-target substances such as base metals, minerals, and alloys.

Example passivating agents include silicate materials and phosphate materials. Example silicate materials include sodium silicate and silicic acid. Example phosphate materials include phosphoric acid and phosphate salts, such as sodium phosphate. Other example passivating agents include diethylenetriamine (DETA) and triethylenetetramine (TETA).

An example preferred passivating agent is phosphoric acid. Without being bound by a particular theory, it is speculated that phosphoric acid not only provides an acidic component to the leaching solutions but also acts as a passivating agent for metals. Phosphoric acid passivates various metal surfaces, particularly iron (Fe) surfaces.

In addition, phosphoric acid appears to limit base metals from being in solution. It is speculated that this is because phosphoric acid forms phosphates with base metals and many phosphates are insoluble. For example, copper, iron, and zinc phosphates are insoluble.

C. Leaching Techniques: Precious Metal Solubility

Upon contact with substrate containing precious metal, the leaching solution solubilizes the precious metal. Contact time between the leaching solution and the substrate can be selected to achieve desired recovery targets and processing goals. The addition of weak acid, such as phosphoric acid or water-soluble, carboxylic acid, to the leaching solution improves the ability of the instant leaching solutions to solubilize precious metal.

D. Leaching Techniques: Storage Stability

After leaching precious metal from the substrate, the pregnant leach solution can be stored or transported. Days, weeks, or months can pass before the precious metals are extracted from pregnant leach solutions. The instantly-disclosed leaching solutions include additive that improve the storage stability of the precious metal.

In preferred processes, the precious metal stays in solution during a storage period without requiring cyanide materials, termed “storage stable.” Herein, “storage stable” in this context means that the pregnant solution can stand for at least an hour without agitation and without the precious metal falling out of the pregnant solution. Pregnant leaching solutions disclosed herein are preferably storage stable for days. Often, pregnant leaching solutions disclosed herein are storage stable for weeks. In some instances, pregnant leaching solutions disclosed herein are storage stable for months.

The pregnant leaching solutions disclosed herein are storage stable under typical conditions, such as ambient temperature and pressure. Without being bound by a particular theory, it is speculated that carboxylic acid material and/or boric acid enhances the stability of the pregnant leach solution, such that the pregnant leach solution can be stored for weeks, months, or longer.

E. Preferred Avoidance of Components in the Leaching Solution

Although strong acids can be used, the instantly-disclosed leaching solutions preferably avoid the use of strong acids. For instance, harsh acids are generally avoided. As an example, sulfuric acid (having pKa values of −3 and 1.99) is preferably avoided. That said, strong acids such as sulfuric acids and hydrochloric acids can be used in addition to the weak acids discussed above.

F. Electrochemical Cell

One or more electrochemical cells are used during typical leaching operations. As discussed in greater detail above, use of electrochemical cells includes maintaining the oxidation reduction potential of leaching solutions and regenerating barren solution. In implementations where more than one electrochemical cell is used, the electrochemical cells may be the same or different. In some instances, divided electrochemical cells are used. Alternatively, electrochemical cells having conductive diamond electrodes are used.

Diamond is normally insulating but can be made conducting by doping it with certain elements. Most commonly, diamond electrodes are doped with boron. Other types of conductive diamond electrodes, doped and undoped, have also been developed and can be used in conjunction with this disclosure. The Diamox cell (Element Six Technologies, Santa Clara, Calif.) is an example commercially-available electrochemical cell utilizing solid boron doped diamond (BDD) electrodes. Without being bound by a particular theory, utilizing electrochemical cells with conductive diamond electrodes results in achieving target ORP values in much shorter time periods as compared to electrochemical cells utilizing various metal or carbon electrodes.

G. Ion Exchange System

One or more ion exchange systems can be used during leaching operations. Generally, ion exchange resins are utilized to obtain protons from more common and/or cheaper acids, such as sulfuric acid, hydrochloric acid, and nitric acid. A commercial embodiment of an ion exchange system is the Acid Purification Unit manufactured by Eco-Tec (Pickering, Ontario, Canada).

In an example implementation, protons are loaded onto an ion exchange resin by passing sulfuric acid, H₂SO₄, through the ion exchange system. Then, leaching solution is passed through the ion exchange system to strip off the H⁺ ions that were loaded onto the ion exchange resin. As a result, it is speculated that the leaching solution leaving the ion exchange system includes iodide material and triiodide ions. In some implementations, a reaction takes place following contact between leaching solution and ion exchange resin as follows:

IO₃ ⁻+2I⁻+6H⁺→I₃ ⁻+3H₂O

The resulting leaching solution can then be subjected to electrochemical treatment with an electrochemical cell.

II. Example Processing

A. General Process

FIG. 1 is an example embodiment of method 10 of recovering precious metal using leaching solution. The method 10 includes obtaining leaching solution (operation 20), leaching (operation 30), and recovering (operation 40). FIG. 2 shows a more detailed method 100 for recovering precious metal using leaching solutions and is discussed below.

The example method 10 begins by obtaining leaching solution (operation 20). The leaching solution can be pre-made and obtained from a third party or prepared on-site. Generating the leaching solution includes preparing an aqueous based solution and raising the oxidation-reduction potential (ORP) of the solution. As part of preparing the aqueous-based solution, one or more additives are added. Each additive is present in an amount sufficient to oxidize, solubilize, and/or stabilize the precious metal. Usable additives and methods of preparing the leaching solution are discussed below in more detail.

After the leaching solution is obtained (operation 20), substrate containing precious metal is contacted with the leaching solution (operation 30). Leaching (operation 30) strips precious metal from the substrate and forms complexes such that the precious metal is in solution, creating a pregnant solution. When the precious metal is in solution, then the solid substrates can be separated from the pregnant solution. Removing solids from the pregnant solution creates a clear solution that improves the efficacy of precious metal recovery steps.

Next, precious metal is recovered from the solution (operation 40). Recovering precious metal (operation 40) can include one or more operations. Precious metal recovery operations can include methods such as electrowinning, precipitation, cementation or loading onto activated carbon, and/or ion exchange resins, or any combination thereof.

Recovery (operation 40) can also include recovering one or more additives by an electrolysis step and/or reactivating the leaching solution. The leaching solution is then reused for subsequent leaching.

A non-limiting example of a method for recovering a precious metal from a precious metal-containing substrate can include the steps of (i) combining the precious metal-containing substrate with any aqueous-based leaching solution disclosed herein (i.e., containing phosphoric acid) to form solids and a pregnant leach solution, and (ii) separating the solids from the pregnant leach solution. Prior to step (i), the aqueous-based leaching solution can be passed through an electrochemical cell to reach an oxidation reduction potential (ORP) of at least 540 mV, such as from 540 mV to 650 mV, from 570 mV to 620 mV, or from 580 mV to 600 mV, and the like.

If desired, the method can further comprise the steps of (iii) removing the precious metal from the pregnant leach solution, thereby forming a barren leaching solution, and (iv) increasing the oxidation reduction potential (ORP) of the barren leaching solution to at least 540 mV to form a regenerated aqueous-based leaching solution.

Beneficially, the use of phosphoric acid in the leaching solution leads to significant and unexpected reductions in the amounts of certain metals—e.g., iron, nickel, copper, and zinc—present in the pregnant leach solution. While not limited thereto, the pregnant leach solution can contain less than 4 mg/L of iron, less than 3.5 mg/L of iron, less than 3 mg/L of iron, less than 2 mg/L of iron, or less than 0.5 mg/L of iron; additionally or alternatively, less than 2 mg/L of nickel, less than 1 mg/L of nickel, or less than 0.8 mg/L of nickel; additionally or alternatively, less than 450 mg/L of copper, less than 350 mg/L of copper, less than 200 mg/L of copper, less than 100 mg/L of copper, less than 20 mg/L of copper, or less than 10 mg/L of copper; additionally or alternatively, less than 25 mg/L of zinc, less than 15 mg/L of zinc, or less than 10 mg/L of zinc. As one of skill in the art would recognize, the amounts of these metals can vary based on the source of the precious metal-containing substrate and the build-up of the metals over multiple uses of the leaching solution.

Unexpectedly, the methods disclosed herein can produce a pregnant leach solution containing less iron (and/or less nickel, and/or less copper, and/or less zinc) than a pregnant leaching solution produced by an otherwise identical process conducted with an aqueous leaching solution containing a carboxylic acid instead of phosphoric acid. This is meant to indicate that the same metal substrate is used, the same ORP, the same iodide and iodate and/or chlorite components are used and their respective amounts, etc., such that the only difference is the use of the carboxylic acid (e.g., citric acid) instead of phosphoric acid. Thus, the disclosed methods can produce a pregnant leach solution having an iron content (and/or a nickel content, and/or a copper content, and/or a zinc content) that is at least 25% less than, at least 50% less than, at least 75% less than, at least 85% less than, or at least 95% less than, that of a pregnant leaching solution produced by an otherwise identical process conducted with an aqueous leaching solution containing a carboxylic acid instead of phosphoric acid. As an example, if the zinc content of the pregnant leach solution is 4 mg/L using phosphoric acid, and the zinc content of the pregnant leach solution is 36 mg/L using citric acid, then the percentage decrease is equal to 88% (36 mg/L minus 4 mg/L, and the result divided by 36 mg/L, and multiplied by 100).

B. Example Processes for Producing Leaching Solution and Recovering Precious Metal

FIG. 2 is an example embodiment of method 100 for generating and using aqueous-based leaching solution. The method 100 shown includes mixing in tank (operation 102), passing solution through electrochemical (EC) cell (operation 104), receiving ore (operation 108), reducing ore size (operation 110), mixing reduced-in-size ore with leachate (operation 112), leaching (operation 114), removing solids (operation 116), recovering gold (operation 118), secondary gold recovery (operation 120), and leach solution recycle (operation 122). Other embodiments can include more or fewer operations.

The embodiment of method 100 begins by obtaining the leaching solution (operation 20). As shown in FIG. 2, the leaching solution is obtained by generating the solution on-site. Specifically, obtaining the leaching solution (operation 20) includes combining ingredients in an aqueous solution in a tank (operation 102) and then preferably passing the solution through an electrochemical cell (operation 104). When in solution, or after current is passed through the solution, the ingredients produce one or more oxidants.

Generally, iodide material and phosphoric acid are combined with water and then agitated or mixed during operation 102. Optionally, iodate material and/or chlorite material is also included. For example, potassium iodide, potassium iodate and/or sodium chlorite, and citric acid are combined with water in a stirred tank during operation 102. Other additives, such as those discussed within this disclosure (e.g., bromide salt, passivating agents), can also be added to the aqueous mixture.

The tank's contents can be agitated or stirred using a baffle, stirrer, or other apparatus designed to promote mixing of tank contents. The amount of ingredients and the tank size can be scaled to accommodate desired precious metal recovery requirements. Specific examples of ingredients and their relative amounts are provided below in the Example section.

After the ingredients are mixed, the solution is directed to an electrified cell (operation 104). For instance, the solution prepared during operation 104 is passed through an electrochemical cell. The electrochemical cell can be divided or undivided. Additionally, more than one electrochemical cell can be used, where the cells are arranged in series and/or in parallel. Typical commercially-available electrochemical cells can be used during operation 104. In some embodiments, electrochemical cells used during operation 104 utilize conducting diamond electrodes.

While passing solution through an electrochemical cell for a given period of time (operation 104), the ORP of the solution is monitored. Preferably, the solution is electrified until the ORP is raised to at least 540 mV SHE (standard hydrogen electrode), such as in a range from 540 mV to 650 mV. More preferably, the solution is electrified until the ORP is raised to at least 570 mV SHE, such as in a range from 580 mV to 600 mV. The resulting mixture can be used as leaching solution and stored in a separate tank until it is used in operation 112, discussed below.

Method 100 also includes receiving ore (operation 108) containing precious metal. The ore is obtained from one or more mining operations that can be local or remote. The ore in the embodiment of method 100 includes gold, although the method 100 can be used to recover other types of precious metal. In other implementations of method 100, electronic waste is processed instead of mined ore.

Although the received ore can be crushed, in some embodiments, the ore is further reduced in size (operation 110). Reducing the ore size can include passing the mined ore through a mill. An example mill is described in U.S. patent application Ser. No. 15/063,725, titled “MILL.”

When the ore is reduced in size and the leaching solution is prepared, both are combined (operation 112). Mixing the reduced-in-size ore with the leaching solution (operation 112) is preferably a batch process, although mixing is optionally performed as a continuous process.

Then the leaching solution and ore undergo leaching (operation 114). Preferably, leaching (operation 114) is conducted as a tank leach. More preferably, leaching (operation 114) is conducted as an agitated tank leach. In other embodiments, vat leaching, heap leaching, and/or in-situ leaching can be used in addition to, or in place of, tank leaching. Tank leaching (operation 114) proceeds at ambient temperature and atmospheric pressure for a predetermined period of time. In various implementations, leach time can vary from a few minutes to over 24 hours, depending upon the materials involved in the process.

During tank leaching (operation 114), gold from the ore goes into solution, resulting in pregnant leach solution. The pregnant leach solution also includes ore solids and the substrate that are preferably removed before the gold recovery operation(s). Optionally, during tank leaching (operation 114), some of the slurry is continuously passed through the electrochemical cell of operation 104 to maintain a desired ORP (loop 106).

After leaching, the solids are removed (operation 116) through one or more operations, resulting in a pregnant leach solution. For example, removing solids (operation 116) preferably includes filtering or clarification, followed by filtering the underflow. Solids removed during operation 116 can also be subjected to a wash to recover any entrained pregnant leach solution. Other solids removal operations can include fewer or more processes.

When most or all of the solids above a predetermined size are removed from the pregnant solution, gold is recovered (operation 118). Preferably, all solids are removed from the pregnant solution before gold recovery (operation 118). In some instances, the pregnant solution is stored and/or transported to a different location before gold recovery. Gold recovery (operation 118) can include electroplating/electrowinning, precipitation, cementation, ion-exchange, and/or adsorption onto activated carbon, to extract the gold out of solution. The gold recovery operation (118) is preferably performed as a continuous process. However, gold recovery (operation 118) is optionally conducted as a batch process.

In some instances, electroplating (operation 118) cannot remove all the gold from solution. Thus, the embodiment of method 100 optionally includes one or more secondary gold recovery operations (120). Secondary gold recovery operations (120) take advantage of other properties of the precious metal to remove precious metal from solution.

Secondary gold recovery (operation 120) preferably includes passing the solution through one or more resin columns having ion exchange resin. An example ion exchange resin is anionic resin beads. Secondary gold recovery (operation 120) optionally includes, in place of or in addition to, passing the solution over activated carbon. Other secondary gold recovery operations are possible.

The barren solution from the gold recovery (operation 118) and secondary gold recovery (operation 120) are next subjected to leach solution recycle (operation 122). Leach solution recycle (operation 122) includes passing the solution through one or more electrochemical cells. That is, one or more cells having cathode(s) and anode(s). Leach solution recycle (operation 122) can also include adding or replenishing one or more of the additives added during operation 102 to bring each additive's concentration to a desired concentration amount.

The ORP of the solution can be monitored and used in determining the residence time of the solution or the flow rate of the solution through the electrochemical cell(s). Without being bound by a particular theory, passing the solution through the electrochemical cells is believed to reactivate/regenerate iodine, as well as other oxidants, in the solution.

In situations where gold is precipitated at a high pH, leach solution recycle (operation 122) can additionally include readjusting the pH of the leach solution. Lowering the pH is preferably performed before passing the leach solution through the electrochemical cell. Additionally, lowering the pH can raise the ORP back up to near target levels before the solution is regenerated in the (preferably divided) electrochemical cell. In some embodiments, electrochemical cell(s) including conductive diamond electrodes are used during regeneration.

The solution with the regenerated contents is then re-used and mixed with the reduced-in-size ore (operation 112).

FIG. 9 is an example embodiment of method 200 for generating and using aqueous-based leaching solution. The method 200 shown differs from method 100 primarily by the addition of operation 203, passing solution through ion exchange system. Otherwise, the corresponding, contemplated components and operations are largely the same between method 100 and method 200. Example method 200 is applicable to precious-metal containing substrate, particularly electronic waste, ores, and the like.

Passing solution through ion exchange system (operation 203) typically occurs after leaching components are mixed in tank (operation 202). In some implementations, barren leaching solution is passed through ion exchange system (operation 203) from leaching (operation 214). After passing solution through the ion exchange system (operation 203), solution is passed through the electrochemical cell (operation 204).

By passing solution through ion exchange system (operation 203), the solution acquires protons (H⁺ ions). The ion exchange system includes strong acid resin. The strong acid resin can be replenished by cheap strong acids, such as sulfuric acid, hydrochloric acid, and nitric acid. Replenishing the ion exchange resin includes separate operations not shown in FIG. 9.

III. Typical Materials and Preferred Amounts

The leaching solution is typically prepared by mixing weak acid material and iodide material with water. Iodate material and/or chlorite material can also be included in the mixture. Typically, iodate material, chlorite material, and iodide material are added as sodium or potassium salts (e.g., potassium iodide, sodium chlorite, and sodium iodate). Preferably, weak acid material includes phosphoric acid. Water-soluble carboxylic acid can be used in addition to phosphoric acid, such as citric acid. Optionally, acetic acid is used as water-soluble, carboxylic acid. Alternatively, water soluble carboxylic acid includes both citric acid and acetic acid. Boric acid can be included in addition to, or instead of, citric acid and/or acetic acid.

Iodide material concentration in the aqueous solution is at least 1 g/L and typically at least 5 g/L. Usually, iodide material concentration is not greater than 200 g/L. In some instances, iodide material concentration is 5-90 g/L inclusive. In some instances, iodide material concentration is 10-50 g/L inclusive. In other instances, iodide material concentration is 5-40 g/L inclusive. In some instances, iodide material is 20-80 g/L inclusive. Optionally, iodide material concentration is 15-30 g/L inclusive.

The resulting solution is then subjected to electrical current from one or more electrodes for a period of time. Typically, the electrodes are included as part of a commercially-available electrochemical cell. Generally, a variety of electrode materials can be used with the methods and materials disclosed herein. Example electrodes include stainless steel electrodes, combination stainless steel and titanium electrodes, and carbon electrodes. In some embodiments, electrodes are conductive diamond electrodes.

Voltages applied to the electrodes can vary depending upon the nature and design of a given electrochemical cell. Usually, conductive diamond electrodes can deliver about 10 times more current per unit area than conventional electrodes. In some implementations, leaching solution passes through an electrochemical cell. In other implementations, electrodes are introduced to a tank including the leaching solution. In those implementations, voltage is applied, typically, for 1-60 minutes, inclusive. Other durations are contemplated.

Typically, phosphoric acid concentration is at least 1 g/L. Preferably, phosphoric acid concentration is 1-50 g/L inclusive. In some instances, phosphoric acid concentration is 1-40 g/L inclusive. In some instances, phosphoric acid concentration is 10-35 g/L inclusive. In other instances, phosphoric acid concentration is 5-20 g/L inclusive. In some instances, phosphoric acid is 1-20 g/L inclusive. Optionally, phosphoric acid concentration is 15-30 g/L inclusive.

Typically, water soluble, carboxylic acid concentration is at least 1 g/L. Preferably, water soluble, carboxylic acid concentration is 1-50 g/L inclusive. In some instances, water soluble, carboxylic acid concentration is 10-35 g/L inclusive. In other instances, water soluble, carboxylic acid concentration is 5-20 g/L inclusive. Optionally, water soluble, carboxylic acid concentration is 15-30 g/L inclusive.

When present, iodate material concentration is at least 0.1 g/L, typically at least 1 g/L, but no more than 50 g/L. In some instances, when present, iodate material concentration is 5-25 g/L inclusive. In other instances, when present, iodate material is 10-20 g/L inclusive. In other instances, when present, iodate material is 1-40 g/L inclusive. In still other instances, when present, iodate material is 1-20 g/L inclusive.

When present, chlorite salt material concentration is at least 0.1 g/L, typically at least 1 g/L, but no more than 50 g/L. In some instances, when present, chlorite salt material concentration is 5-25 g/L inclusive. In other instances, when present, chlorite salt material is 10-20 g/L inclusive. In other instances, when present, chlorite salt material is 1-40 g/L inclusive. In still other instances, when present, chlorite salt material is 1-20 g/L inclusive.

Boric acid is optionally added to the solution with the complexing agent. Typically, when present, boric acid concentration is at least 0.1 g/L but no more than 20 g/L. Preferably, when present, boric acid concentration is 0.5-3.5 g/L inclusive. In some instances, boric acid concentration is 1.5-8.0 g/L inclusive. Optionally, boric acid concentration is 2.0-5.0 g/L inclusive. Optionally, boric acid is 5.0-15 g/L inclusive.

Preferably, the leaching solutions disclosed herein have no cyanide (e.g. NaCN) added.

IV. Experimental Examples

FIGS. 3-8 show results of laboratory analyses of different leachate solutions and various substrates. The following examples are illustrative and other embodiments are within the scope of the present disclosure.

A. Test Set 1

FIGS. 3-4 illustrate results of laboratory analyses of fourteen different leachate solutions, labeled 3 a-3 n, contacted with froth flotation gold concentrate from a mine located in British Columbia, Canada. FIG. 3 is a bar chart showing gold parts per million in solution after 1 hour of leaching. FIG. 4 is a plot showing the ppm of gold in solution for the fourteen leachate solutions in FIG. 3 over a period of time greater than 1 hour, with measurements recorded every 60 minutes. The results are presented in Table 1 below. Leachate solutions 3 l, 3 n, and 3 m are discussed below in Examples 1, 2 and 3, respectively.

TABLE 1 Concentration of gold in solution (in parts per million) after leaching for 1-6 hours. Mixture Boric Gold in solution (ppm) Citric Acid 1 2 3 4 5 6 Label Acid (g/L) (g/L) hour hours hours hours hours hours 3a 0 0 16 26 30 28 39 3b 0 2 17 24 32 29 41 3c 2 7 32 35 41 43 48 50 3d 5 0 35 50 48 52 64 3e 0 2 35 44 45 49 50 60 3f 5 2 36 47 53 50 64 3g 2 2 41 39 46 49 52 55 3h 7 2 42 47 56 60 62 64 3i 1.3 0 43 56 60 64 68 72 3j 20 2 43 45 47 49 51 54 3k 7 7 46 50 59 64 66 69 3l 10 0 54 56 60 65 70 73 3m 15 2 56 60 64 68 72 74 3n 10 2 63 74 76 78 80 81

The following examples are illustrative and other embodiments are within the scope of the present disclosure.

1. Example 1—Leachate Solution 3 l

An example embodiment of the leaching solution was prepared and the gold recovery analyzed. The leaching solution was prepared by adding 25 g KI and 3 g NaCl to 1 L of tap water in a 2 L beaker. The contents were agitated with a magnetic stirrer. Then, a stainless steel cathode and a titanium anode were placed into the solution. The electrodes were electrified at 6 V (0.5 amps) for 15 minutes. Then, 10 g of citric acid, C₆H₈O₇, were added and the contents stirred using the magnetic stirrer. The resulting solution was the leaching solution.

Next, 100 mL of the leaching solution was added to 33.3 g of test material in a 500 mL bottle container. The test material was froth flotation gold concentrate. The 500 mL bottle container was placed on rollers that provide constant agitation.

Every 60 minutes, the bottle was opened and 1 mL of fluid was drawn out and combined with 6 mL of deionized water in a 16 mL test tube. The test tube was placed into a centrifuge for 2 minutes at 1,000 rpm, which accelerated the settling of the solids.

Then gold concentration of the clarified solution in the test tube was measured using an atomic absorption spectroscopy machine. Two measurements were recorded. From the measurements obtained, the amount of gold in solution was determined in terms of parts per million (ppm). After 60 minutes, gold was present at 54 ppm.

A leachate solution with 0 g citric acid (the “standard”) had 16 ppm gold after one hour. Thus, the leaching solution of Example 1 recovered 38 ppm more than the standard solution, an improvement of 238%. Without being bound by a particular theory, the data in FIGS. 3-5 appear to suggest that citric acid improves gold recovery/solubility in solution.

2. Example 2—Leachate Solution 3 n

An example embodiment of the leaching solution was prepared and the gold recovery analyzed. The leaching solution was prepared by adding 25 g KI and 3 g NaCl to 1 L of tap water in a 2 L beaker. The contents were agitated with a magnetic stirrer. Then, a stainless steel cathode and a titanium anode were placed into the solution. The electrodes were electrified at 6 V (0.5 amps) for 15 minutes. Then, 10 g of citric acid, C₆H₈O₇, and 2 g of boric acid, H₃BO₃, were added and the contents stirred using the magnetic stirrer. The resulting solution was the leaching solution.

Next, 100 mL of the leaching solution was added to 33.3 g of test material in a 500 mL bottle container. The test material was froth flotation gold concentrate. The 500 mL bottle container was placed on rollers that provide constant agitation.

Every 60 minutes, the bottle was opened and 1 mL of fluid was drawn out and combined with 6 mL of deionized water in a 16 mL test tube. The test tube was placed into a centrifuge for 2 minutes at 1,000 rpm, which accelerated the settling of the solids.

Then the gold concentration of the clarified solution in the test tube measured using an atomic absorption spectroscopy machine. Two measurements were recorded. From the measurements obtained, the amount of gold in solution was determined in terms of parts per million (ppm). After 60 minutes, gold was present at 63 ppm.

Thus, the leaching solution of Example 2 had 47 ppm more gold in solution than the standard solution. This is an improvement of 294% over the standard solution. Without being bound by a particular theory, the addition of boric acid to the leaching solution of Example 1 improved the gold recovery.

3. Example 3—Leachate Solution 3 m

An example embodiment of the leaching solution was prepared and the gold recovery analyzed. The leaching solution was prepared by adding 25 g KI and 3 g NaCl to 1 L of tap water in a 2 L beaker. The contents were agitated with a magnetic stirrer. Then, a stainless steel cathode and a titanium anode were placed into the solution. The electrodes were electrified at 6 V (0.5 amps) for 15 minutes. Then, 15 g of citric acid, C₆H₈O₇, and 2 g of boric acid, H₃BO₃, were added and the contents stirred using the magnetic stirrer. The resulting solution was the leaching solution.

Next, 100 mL of the leaching solution was added to 33.3 g of test material in a 500 mL bottle container. The test material was froth flotation gold concentrate. The 500 mL bottle container was placed on rollers that provide constant agitation.

Every 60 minutes, the bottle was opened and 1 mL of fluid was drawn out and combined with 6 mL of deionized water in a 16 mL test tube. The test tube was placed into a centrifuge for 2 minutes at 1,000 rpm, which accelerated the settling of the solids.

Then the gold concentration of the clarified solution in the test tube was measured using an atomic absorption spectroscopy machine. Two measurements were recorded. From the measurements obtained, the amount of gold in solution was determined in terms of parts per million (ppm). After 60 minutes, gold was present at 56 ppm.

Thus, the leaching solution of Example 3 had 40 ppm more gold in solution than the standard solution. However, the recovery of the leaching solution of Example 3 was less than Example 2. Without being bound by a particular theory, the addition of 5 g more citric acid to the leaching solution of Example 2 did not improve gold recovery. This is confirmed by the test leaching solution having 20 g citric acid and 2 g boric acid, which had 43 ppm gold after 1 hour, less than each of Examples 1, 2 and 3.

B. Test Set 2

FIGS. 5-8 are plots showing gold recovered into solution for different leachate solutions expressed as a percentage, with various substrates, over a period of time.

1. Example 1—Froth Flotation Concentrate

Various leaching solutions were prepared and the gold recovery from froth flotation concentrate analyzed. The results of gold recovery with various leaching solutions are shown in FIG. 5 and in Table 2 below.

TABLE 2 Gold recovery percentage for various test leaching solutions over time for froth flotation concentrate sample. Additive Gold Recovery (%) after Conc. 3 24 Test Additive Type (g/L) 1 hour hours 6 hours hours IB321 Citric acid 10 43.2 59.0 66.7 78.9 IB322 Acetic acid 20 41.4 58.8 66.5 80.4 IB323 Boric acid 20 33.4 47.9 60.1 77.8 IB281 Boric acid 0.5 38.9 53.4 64.8 75.7 AH103 Citric acid/Iodate 5.0/0.5 72.0 75.4 77.9 82.2 AH104 None 0 28.8 40.1 47.6 62.8

Each leaching solution was prepared by mixing tap water with 40 g/L KI and 1.5 g/L NaCl. All but one test solution had one or more additional additives added to the water/KI/NaCl mixture. For test solution IB321, 10 g/L citric acid was also added. For test solution IB322, 20 g/L acetic acid was also added. For test solution IB323, 20 g/L boric acid was also added. For test solution IB281, 0.5 g/L boric acid was also added. For test solution AH103, 5.0 g/L citric acid and 0.5 g/L iodate were also added. For test solution AH104, no additional additives were added to the KI, NaCl, and water mixture. The contents of each test solution were agitated with a magnetic stirrer.

After mixing, each solution was passed through an electrochemical cell to generate a leaching solution. Froth flotation concentrate sample was then mixed with the test leaching solution at a pulp density of 25%. In this example, froth flotation concentrate was produced from mined ore from British Columbia, Canada.

The resulting slurry (combination of froth flotation concentrate and leaching solution) was then continuously leached in an agitated vessel for 24 hours. Samples of the slurry were obtained and assayed at increments of 1, 3, 6 and 24 hours.

After 24 hours, the solids were separated from the solution and washed. Solution samples were placed in test tubes. Then the gold concentration in each test tube was measured using an atomic absorption spectroscopy machine. From the measurements obtained, the amount of gold in solution was determined in terms of parts per million (ppm). The recovery of gold was calculated for each sample. Results of the calculated gold recoveries are shown in FIG. 5.

Without being bound by a particular theory, the results in FIG. 5 show that addition of additive (citric acid, acetic acid, boric acid, and/or iodate) improves the gold recovery of the leaching solution as compared to no additives. The results in FIG. 5 also show that citric acid and acetic acid provide similar gold recoveries over 24 hours. The results in FIG. 5 also show that boric acid performs slightly worse than citric acid and boric acid. The results in FIG. 5 show that the best recoveries were achieved with the addition of citric acid and potassium iodate. Iodate also provided faster leach rates.

2. Example 2—Gravity Concentrate

Various leaching solutions were prepared and the gold recovery from gravity concentrate analyzed. The results of gold recovery with various leaching solutions are shown in FIG. 6 and in Table 3 below.

TABLE 3 Gold recovery percentage for various test leaching solutions over time for gravity concentrate sample. Gold Recovery (%) after Additive Additive 1 6 Test Type Conc. (g/L) hour 3 hours hours 24 hours IB271 None 0 29.0 46.4 55.3 57.1 IB235 Boric acid 0.5 54.6 80.8 85.1 92.5 IB282 Boric acid/ 0.5/10  10.8 71.8 81.5 97.5 Citric acid IB283 Citric acid 10 74.1 84.4 88.5 97.6 IB291 Boric acid/ 20/20 65.8 76.0 84.2 98.1 Citric acid

Each leaching solution was prepared by mixing tap water with 40 g/L KI and 1.5 g/L NaCl. All but one test solution had one or more additional additives added to the mixture. For test solution IB271, no additional additives were added to the KI, NaCl, and water mixture. For test solution IB235, 0.5 g/L boric acid was also added. For test solution IB282, 0.5 g/L boric acid and 10 g/L citric acid were also added. For test solution D3283, 10 g/L citric acid was also added. For test solution D3291, 20 g/L boric acid and 20 g/L citric acid were also added. The contents of each test solution were agitated with a magnetic stirrer.

After mixing, each solution was passed through an electrochemical cell to generate a leaching solution. Gravity concentrate sample was then mixed with the test leaching solution at a pulp density of 25%. In this example, gravity concentrate was obtained from a centrifugal gravity gold concentrator operating at a mine in Arizona.

The resulting slurry (combination of gravity concentrate and leaching solution) was then continuously leached in an agitated vessel for 24 hours. Samples of the slurry were obtained and assayed at increments of 1, 3, 6 and 24 hours.

After 24 hours, the solids were separated from the solution and washed. Solution samples were placed in test tubes. Then the gold concentration in each test tube was measured using an atomic absorption spectroscopy machine. From the measurements obtained, the amount of gold in solution was determined in terms of parts per million (ppm). The recovery of gold was calculated for each sample. Results of the calculated gold recoveries are shown in FIG. 6.

Without being bound by a particular theory, the results in FIG. 6 show that addition of additive (citric acid or boric acid) improves the gold recovery of the leaching solution as compared to no additives. The results in FIG. 6 also show that gold recovery improved with citric acid as the additive as compared to boric acid. The results in FIG. 6 also show that each solution with citric acid performed relatively similarly with respect to gold recovery.

3. Example 3—E-Waste

Various leaching solutions were prepared and the gold recovery from e-waste sample analyzed. The results of gold recovery with various leaching solutions over time are shown in FIG. 7 and in Table 4 below.

TABLE 4 Gold recovery percentage for various test leaching solutions over time for e-waste sample. Additive Gold Recovery (%) after Conc. 60 90 120 Test Additive Type (g/L) 30 Min Min Min Min J101 None  0 44.3 38.6 28.2 18.4 J102 Iodate/Citric acid 10/10 70.6 81.4 82.9 84.9 J103 Citric acid 10 57.2 69.1 75.0 79.7 J104 Boric acid/Citric acid  2/10 58.7 72.8 80.5 83.2

Each leaching solution was prepared by mixing tap water with 60 g/L KI. All but one test solution had one or more additional additives added to the water/KI mixture. For test solution J101, no additional additives were added to the KI and water mixture. For test solution J102, 10 g iodate and 10 g citric acid were also added to the beaker. For test solution J103, 10 g citric acid was also added to the beaker. For test solution J104, 2 g boric acid and 10 g citric acid were also added to the beaker. The contents of each test solution were agitated with a magnetic stirrer.

After mixing, each solution was passed through an electrochemical cell to generate a leaching solution. E-waste sample was then mixed with the test leaching solution at a pulp density of 25%. In this example, e-waste sample included ground electronic waste (such as circuit boards) passed through a 70 mesh screen.

The resulting slurry (combination of e-waste material and leaching solution) was then continuously leached in an agitated vessel for 120 minutes. Samples of the slurry were obtained and assayed at increments of 30, 60, 90 and 120 minutes.

After 120 minutes, the solids were separated from the solution and washed. Solution samples were placed in test tubes. Then the gold concentration in each test tube was measured using an atomic absorption spectroscopy machine. From the measurements obtained, the amount of gold in solution was determined in terms of parts per million (ppm). The recovery of gold was calculated for each sample. Results of the calculated gold recoveries are shown in FIG. 7.

Without being bound by a particular theory, the results in FIG. 7 show that not including an additive in the leaching solution was detrimental to the stability of gold in solution. Including additives appears to prevent gold from precipitating out of solution. The results in FIG. 7 also suggest that solutions with boric acid and citric acid had similar gold recoveries, with leaching solutions including citric acid performing only slightly better. The results in FIG. 7 also show that the best gold recovery was achieved with the addition of both citric acid and potassium iodate to the leaching solution.

4. Example 4—E-Waste Pilot Plant

A leaching solution was prepared and the gold recovery from e-waste sample analyzed. This example utilized a pilot plant as described in the section below titled “Third Example Hypothetical Process”. The results of gold recovery with the leaching solution over time are shown in FIG. 8 and in Table 5 below.

TABLE 5 Gold recovery percentage for a test leaching solution in a pilot plant over time for e-waste sample. Additive Conc. Gold Recovery (%) after Conc. 90 Test Additive Type (g/L) 30 Min 60 Min Min 120 Min Pilot Citric/Iodate 10/10 66.4 76.5 81.0 83.4

The leaching solution was prepared by adding 50 g/L KI, 10 g/L citric acid, and 10 g/L potassium iodate to 100 L of tap water. The contents of the test solution were agitated with a magnetic stirrer.

After mixing, each solution was passed through a Diamox electrochemical cell (the cell utilizing conductive diamond electrodes) to generate a leaching solution having an ORP of 590 mV (SHE). E-waste sample was then mixed with the test leaching solution at a pulp density of 14%. In this example, e-waste sample included ground electronic waste (such as circuit boards) passed through a 100 mesh screen.

The resulting slurry (combination of gravity concentrate and leaching solution) was then continuously passed through the Diamox electrochemical cell to maintain the ORP at or above 590 mV (SHE). The resulting slurry was then continuously leached in an agitated vessel for 120 minutes. Samples of the slurry were obtained and assayed at increments of 30, 60, 90 and 120 minutes.

After 120 minutes, the solids were separated from the solution and washed. Solution samples were placed in test tubes. Then the gold concentration in each test tube was measured using an atomic absorption spectroscopy machine. From the measurements obtained, the amount of gold in solution was determined in terms of parts per million (ppm). The recovery of gold was calculated for each sample. Results of the calculated gold recoveries are shown in FIG. 8.

The results in FIG. 8 show the stability and scalability of the process. In contrast to the examples 1-3 discussed above (those referring to FIGS. 5-7), this is the only example where the slurry was continuously passed through the electrochemical cell to maintain the ORP. Thus, this example proved up the lab-scale processes described with reference to examples 1-3 (results shown in FIGS. 5-7).

5. Example 5—Chlorite Salt

A leaching solution was prepared and the gold recovery from e-waste sample analyzed. Leaching solutions in Test 1 and Test 2, described below, included citric acid and sodium chlorite (Test 1) and acetic acid and sodium chlorite (Test 2). Two cycles were run in teach test. Gold recovery results for the leaching solutions over time, and for each test, are shown in Tables 6 and 7 below.

TABLE 6 Test 1: gold recovery percentage for a test leaching solution over time for e-waste sample. Additive Conc. Gold Recovery (%) after Conc. 45 Cycle # Additive Type (g/l) 15 Min 30 Min Min 60 Min 1 Chlorite/Citric 17/8 61 77 82 84 2 Chlorite/Citric 15/8 67 67 78 81

In Test 1, leach solution was prepared by mixing 40 g/l of NaI, 8 g/l of sodium chlorite and 2 g/l of citric acid. A sample of ground e-waste was then mixed with the leaching solution to a pulp density of 13%. The sample was leached for 1 hour and samples of the solutions were taken at 15, 30, 45 and 60 minute intervals and the gold content was measured for each solution.

Citric acid was added during the leach to maintain the ORP value above 565 mV. After 1 hour, the solids were separated from the solution and washed. The solids were then dried and assayed to determine the gold content remaining in the solids. The gold recovery was calculated for each interval. Calcium hydroxide was added to the pregnant solution to precipitate various metals. The precipitate was then filtered to remove the precipitate.

The filtered pregnant solution was then subject electrochemical regeneration and gold recovery via electrowinning for 30 minutes in an electrochemical cell. The solution was then used for a second test as described above. The leach results from the two cycles as well as the gold recoveries are presented in Table 6.

TABLE 7 Test 2: gold recovery percentage for a test leaching solution over time for e-waste sample. Additive Conc. Gold Recovery (%) after Conc. 45 Cycle # Additive Type (g/l) 15 Min 30 Min Min 60 Min 1 Chlorite/Acetic 10/6.9 80 84 84 84 2 Chlorite/Acetic 10/8.4 67 74 77 84

In Test 2, the leaching solution was prepared by mixing 40 g/l of NaI, 10 g/l of sodium chlorite and 3.3 g/l of acetic acid. A sample of ground e-waste was then mixed with the leaching solution to a pulp density of 13%. The sample was leached for 1 hour and samples of the solutions were taken at 15, 30, 45 and 60 minute intervals and the gold content was measured for each solution.

Acetic acid was added during the leach to maintain the ORP value above 565 mV. After 1 hour, the solids were separated from the solution and washed. The solids were then dried and assayed to determine the gold content remaining in the solids. The gold recovery was calculated for each interval. Calcium hydroxide was added to the pregnant solution to precipitate various metals. The precipitate was then filtered to remove the precipitate.

The filtered pregnant solution was then subject electrochemical regeneration and gold recovery via electrowinning for 30 minutes in an electrochemical cell. The solution was then used for a second test as described above. The leach results from the two cycles as well as the gold recoveries are presented in Table 7.

V. Example Hypothetical Process Using Materials and Techniques Characterized Herein

In a hypothetical precious metal recovery operation, gravity concentrate including precious metal is received from a mining operation. The gravity concentrate is processed to reduce the size in a mill, where the concentrate exiting the output of the mill generally has a size not greater than 2000 μm.

Separately, in an agitated tank, water is combined with potassium iodide, potassium iodate, and citric acid, and agitated. The potassium iodide is added to the tank in an amount of 25 grams per liter. The potassium iodate is added to the tank in an amount of 25 grams per liter. In an alternate implementation, sodium chlorite is added to the tank, in place of the potassium iodate, in an amount of 17 grams per liter. The citric acid is added to the tank in an amount of 10 grams per liter.

Next, the water-additive mixture is pumped to a process tank including electrodes. An ORP meter is positioned such that it can record measurements of the process tank contents.

A voltage of 6 V is provided to each electrode in the process tank. When the ORP meter indicates a predetermined level has been reached, the voltage to the electrodes is stopped. The aqueous solution is enhanced and ready for leaching.

The gravity concentrate from the mill is delivered to an agitated leach tank. Leaching in the agitated leach tank proceeds for a predetermined amount of time, such as, for example, 12 hours.

The leach solution including solids is then pumped through a filter press to separate the solids from the solution.

Solution passing through the filter press is sent to an electrowinning cell. The output from the electrowinning cell is pumped through activated carbon.

Solution passing through the carbon resin is pumped to an electrode-containing cell. The output from the electrode-containing cell is pumped back to the agitated leach tank.

VI. Second Example Hypothetical Process Using Materials and Techniques Characterized Herein

In a hypothetical precious metal recovery operation, mined ore including precious metal is received from a mining operation. The mined ore is processed to reduce the size in a mill, where the ore exiting the output of the mill generally has a size not greater than 250

Separately, in an agitated tank, water is combined with: anywhere between 10 grams per liter to 40 grams per liter potassium iodide, 40 grams per liter sodium iodate, 1.5 grams per liter citric acid monohydrate, 1.5 grams per liter boric acid, and anywhere between 5 mL per liter to 20 mL per liter glacial acetic acid. The combination is then agitated.

Next, the mixture is pumped to a process tank including electrodes. An ORP meter is positioned such that it can record measurements of the process tank contents.

The mixture is subjected to electrolysis using a carbon anode and stainless steel cathode until the oxidation reduction potential (ORP) of the mixture is a minimum of 540 mV SHE, more preferably a minimum of 570 mV SHE. Typical ranges include, but are not limited to, from 540 mV to 650 mV, from 570 mV to 620 mV, or from 580 mV to 600 mV, and the like. Alternatively, platinum or titanium cathodes can be used.

The ore from the mill is delivered to the process tank. Ore is added to the mixture and agitated to create a slurry of 10% to 25% ore volume by weight. Electrolysis then continues for the required leach cycle duration to maintain the ORP levels.

After a predetermined amount of time, for example, 12 hours or 24 hours, the leach solution including solids is then pumped through a filter press to separate the solids from the solution.

Solution passing through the filter press is sent to an electrowinning cell. The output from the electrowinning cell is pumped through activated carbon. Solution passing through the activated carbon is pumped to an electrode-containing cell. The output from the electrode-containing cell is pumped to the back to the agitated leach tank.

VII. Third Example Hypothetical Process Using Materials and Techniques Characterized Herein

In a hypothetical precious metal recovery operation, flotation concentrate solids including precious metal are received from a mining operation.

Separately, in a leaching tank, water is combined with potassium iodide, potassium iodate, and citric acid, and agitated. The potassium iodide is added to the tank in an amount of 40 grams per liter. The potassium iodate is added to the tank in an amount of 10 grams per liter. The citric acid is added to the tank in an amount of 5 grams per liter. The resulting contents are termed a “water-additive mixture”.

An ORP probe is positioned in the leaching tank so that the ORP probe can measure the ORP of the solution.

Next, the water-additive mixture is pumped through an electrochemical cell and returned back to the tank until the measured ORP of the water-additive mixture is raised to a value of at least 570 mV (vs SHE). In some embodiments, the electrochemical cell utilizes conductive diamond electrodes. The aqueous solution is enhanced and ready for leaching.

The concentrate from a mill is delivered to the leach tank to make a slurry with a pulp density of 10-35% by weight. Leaching in the leach tank proceeds for 20 hours. The slurry is continuously pumped through the conductive diamond electrode electrochemical cell to maintain the ORP above 570 mV (SHE).

After a predetermined amount of time, such as 20 hours, the leach solution (pregnant leach solution or “PLS”) including the solids are pumped through a filter. Wash water is pumped through the filter to recover entrained PLS.

Solution and wash water passing through the filter press is sent to a divided electrochemical cell to recover gold by electrowinning. In addition, or alternatively, gold is recovered by precipitation. The solids are discarded.

VIII. Fourth Example Hypothetical Process Using Materials and Techniques Characterized Herein

In a hypothetical precious metal recovery operation, electronic waste solids are received. Separately, in a leaching tank, water is combined with potassium iodide, potassium iodate, and sodium chlorite, and agitated. The potassium iodide is added to the tank in an amount of 40 grams per liter. The potassium iodate is added to the tank in an amount of 1 gram per liter, and the sodium chlorite is added to the tank in an amount of 9 grams per liter (alternatively, the amounts of potassium iodate and sodium chlorite can be reversed, or equal amounts of 5 grams per liter can be used). Then phosphoric acid is added to the tank in an amount of 5 grams per liter. The resulting contents are termed a “water-additive mixture”. An ORP probe is positioned in the leaching tank so that the ORP probe can measure the ORP of the solution.

The water-additive mixture is then passed through an ion exchange system. The water-additive mixture leaving the ion exchange system is pumped through an electrochemical cell and returned back to the tank. Water-additive mixture is cycled between the electrochemical cell and the leaching tank until the measured ORP of the water-additive mixture is raised to a value of at least 570 mV (vs SHE). In some embodiments, the electrochemical cell utilizes conductive diamond electrodes. The aqueous solution is enhanced and ready for leaching.

The electronic waste is delivered to the leach tank to make a slurry with a pulp density of 10-35% by weight. Leaching in the leach tank proceeds for 20 hours. The slurry is continuously pumped through the conductive diamond electrode electrochemical cell to maintain the ORP above 570 mV (SHE).

After a predetermined amount of time, such as 20 hours, the leach solution (pregnant leach solution or “PLS”) including the solids are pumped through a filter. Wash water is pumped through the filter to recover entrained PLS.

Solution and wash water passing through the filter press is sent to a divided electrochemical cell to recover gold by electrowinning. In addition, or alternatively, gold is recovered by precipitation or carbon loading. The solids are discarded.

IX. Example Theoretical Chemistry During Processes and Techniques Characterized Herein

Without being bound by a particular theory, it is theorized that the following reactions take place during one or more of the processes and techniques characterized herein. Gold leaching with iodine/iodide takes place via the following reactions:

2Au+I⁻+I₃ ⁻→2AuI₂ ⁻  (5)

2Au+3I₃ ⁻→2AuI₄ ⁻+I⁻  (6)

The addition of phosphoric acid, carboxylic acids, and/or boric acid has been shown to accelerate the leaching rate and stabilize gold in solution.

The addition of iodate (as, e.g., KIO₃) and an acid to the leaching solution can result in a rapid increase in the ORP level via the following reaction:

IO₃ ⁻+2I⁻+6H⁺→I₃ ⁻+3H₂O  (7)

An electrochemical cell is used to maintain or increase the ORP over and above the ORP achieved from reaction (7) above. The reaction on the anode in the electrochemical cell generates a triiodide species required for leaching according to the following reaction:

3I⁻→I₃ ⁻+2e ⁻  (8)

The reaction at the cathode in the electrochemical cell is:

2H₂O+2e ⁻→H₂+2OH⁻  (9)

When iodine and iodide are present in a system, a result is the formation of the triiodide ion. In the systems disclosed herein, any iodine produced will typically lead to the formation of triiodide because there are typically always iodide ions present in the system

I₂+I⁻=I₃ ⁻  (10)

If chloride (which can be a source of chlorine to produce hypochlorite electrochemically in the cell) or if hypochlorite is added to the system, the following reactions can also take place to produce iodine:

2I⁻+Cl₂→+I₂+2Cl⁻  (11)

Cl₂+2OH⁻→H₂O+Cl⁻+ClO⁻  (12)

ClO⁻+2H⁺+2I⁻→I₂+Cl⁻+H₂O  (13)

Addition of bromides will have a similar effect as that provided above.

If chlorite salt (e.g., as NaClO₂) is added with some, or no, iodate material, the following reaction can take place:

ClO₂ ⁻+4I⁻+4H⁺→2I₂+Cl⁻+2H₂O  (14)

Then, I₂ reacts with excess I— to produce triiodide.

Gold and other metals in solution can be recovered via the various methods described above. However, in the case of e-waste, one preferred route is to raise the pH with the addition of a hydroxide, such as sodium or potassium hydroxide, to precipitate the metals. A result of hydroxide addition to a solution to raise the pH above 12.5 converts much of the iodine species to iodate, as follows:

3I₂+6OH⁻→5I⁻+IO₃ ⁻+3H₂O  (15)

Once the precipitated gold and metals are removed, the resulting clarified solution is referred to as a “barren solution”. The barren solution, which can include iodide and iodate, is then treated for recycle to be used for leaching. One possible way the solution can be treated for recycle is to lower the pH. If the material has been subjected to precipitation, such as when processing e-waste, acid material is added to the barren solution to lower the pH back down below pH 12 to approximately neutral levels. In this approach, triiodide is created as shown in reaction (7) above. A number of different acids, inorganic or organic, can be used in this step (e.g., sulfuric acid, hydrochloric acid, citric acid, boric acid, etc.).

Then the barren solution is treated through a cell with conductive diamond electrodes. Alternatively, the barren solution is treated through a divided electrochemical cell with the leaching solution (anolyte) passing through the anode side of the cell and a hydroxide solution (catholyte) on the cathode side. The reaction on the anode side is shown above as reaction (8). Electrochemical cell treatment is conducted until a desired ORP is achieved. Then the regenerated leaching solution is available for leaching again.

X. Example Using Phosphoric Acid

This example demonstrates the significant and unexpected benefit of using phosphoric acid. Two sets of tests were conducted using two different e-waste samples: a low grade sample (low gold content) and a high grade sample (high gold content). The e-waste source was ground circuit boards.

These experiments were generally conducted in the same manner as that of Test 2 (see also Table 7). In a leaching tank, water was combined with sodium iodide (50 g/L), potassium iodate (10 g/L), and either citric acid or phosphoric acid, and agitated. An ORP probe was positioned in the leaching tank, and the ORP values of the solutions were in the 550-600 mV range.

The e-waste was delivered to the leach tank to make a slurry with a pulp density of 20% by weight. Leaching in the leach tank proceeded for 1 hour.

Table 8 and Table 9 summarize the test results. The phosphoric acid significantly reduced the amounts of certain base metals extracted into the pregnant solution. Beneficially, the pregnant solution was not readily fouled by the presence of base metals such as those shown in Table 8 and Table 9. Copper and iron are particularly problematic as they build up in the pregnant solution over multiple uses of the same solution. While not wishing to be bound by theory, it may be that the phosphoric acid is preventing the leaching of the metals into the pregnant solution, complexing with the metals, or precipitating the metals.

As shown in Table 8 and Table 9, the phosphoric acid tests produced pregnant solutions containing similar gold concentrations as that of citric acid, but the presence of other metals was significantly reduced. Unexpectedly, copper levels (3-5 mg/L) were 100 times less when using phosphoric acid, and iron was effectively eliminated using phosphoric acid.

TABLE 8 Metal concentration in solution after leaching - Low grade sample (Set 1). Metals Concentration Acid in Solution (mg/L) Test # Acid Type (g/L) Au Fe Ni Cu Zn T204 Citric 15 2.7 4.3 2.1 499 42 T205 Phosphoric 12 3.0 0.0 0.6 5 7

TABLE 9 Metal concentration in solution after leaching - High grade sample (Set 2). Metals Concentration Acid in Solution (mg/L) Test # Acid Type (g/L) Au Fe Ni Cu Zn T104 Citric 15 15.3 5.5 2.9 581 36 T105 Phosphoric 12 15.1 0.0 0.4 3.2 4

It will be clear that the compositions and methods described herein are well adapted to attain the ends and advantages mentioned as well as those inherent therein. Those skilled in the art will recognize that the compositions and methods within this specification can be implemented in many manners and as such are not to be limited by the foregoing exemplified embodiments and examples. In this regard, any number of the features of the different embodiments described herein can be combined into one single embodiment and alternate embodiments having fewer than or more than all of the features herein described are possible. Further, while various embodiments have been described for purposes of this disclosure, various changes and modifications can be made which are well within the scope of the present disclosure. Numerous other changes can be made which will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the disclosure.

While compositions and methods are described herein in terms of “comprising” various components or steps, the compositions and methods also can “consist essentially of” or “consist of” the various components or steps, unless stated otherwise. For example, a leaching solution consistent with embodiments of the present invention can comprise; alternatively, can consist essentially of; or alternatively, can consist of; an iodide salt material, an iodate salt material and/or a chlorite salt material, phosphoric acid, and water. Additionally, unless otherwise specified, the terms “a,” “an,” and “the” are intended to include plural alternatives, e.g., at least one.

The “aqueous-based leaching solution” described herein does not depend upon the actual product or composition resulting from the contact or reaction of the initial components of the disclosed or claimed aqueous-based leaching solution, or the fate of the iodide salt, the iodate salt and/or chlorite salt, and the phosphoric acid, after combining these components. Therefore, the “aqueous-based leaching solution” encompasses the initial starting components of the composition, as well as whatever product(s) may result from contacting these initial starting components. As one of skill in the art would readily recognize, the initial components can react to form a triiodide species, as disclosed hereinabove. 

What is claimed is:
 1. A method for recovering a precious metal from a precious metal-containing substrate, the method comprising: (i) combining the precious metal-containing substrate with an aqueous-based leaching solution to form solids and a pregnant leach solution, wherein the aqueous-based leaching solution comprises: an iodide salt material; an iodate salt material, a chlorite salt material, or any combination thereof; and phosphoric acid; and (ii) separating the solids from the pregnant leach solution.
 2. The method according to claim 1, wherein: the precious metal-containing substrate comprises ore, mining tails, electronic waste, or any combination thereof; and the precious metal and the pregnant leach solution comprise gold.
 3. The method according to claim 1, wherein the method further comprises, prior to step (i), passing the aqueous-based leaching solution through an electrochemical cell to reach an oxidation reduction potential (ORP) of at least 540 mV.
 4. The method according to claim 1, wherein the method further comprises, prior to step (i), passing the aqueous-based leaching solution through an electrochemical cell to reach an oxidation reduction potential (ORP) in a range from 540 mV to 650 mV.
 5. The method according to claim 4, wherein the electrochemical cell comprises conductive diamond electrodes.
 6. The method according to claim 1, wherein the method further comprises: (iii) removing the precious metal from the pregnant leach solution, thereby forming a barren leaching solution; and (iv) increasing the oxidation reduction potential (ORP) of the barren leaching solution to at least 540 mV to form a regenerated aqueous-based leaching solution.
 7. The method according to claim 1, wherein the pregnant leach solution contains: less than 350 mg/L of copper; and less than 3 mg/L of iron.
 8. The method according to claim 1, wherein the pregnant leach solution contains an amount of copper that is at least 75% less than a pregnant leaching solution produced by an otherwise identical process conducted with an aqueous leaching solution containing a carboxylic acid instead of phosphoric acid.
 9. The method according to claim 1, wherein the pregnant leach solution contains an amount of iron that is at least 75% less than a pregnant leaching solution produced by an otherwise identical process conducted with an aqueous leaching solution containing a carboxylic acid instead of phosphoric acid.
 10. The method according to claim 1, wherein: the iodide salt material comprises sodium iodide and/or potassium iodide; the iodate salt material comprises sodium iodate and/or potassium iodate; and the chlorite salt material comprises sodium chlorite and/or potassium chlorite.
 11. The method according to claim 1, wherein the aqueous-based leaching solution contains: from 5 to 90 grams per liter of the iodide salt material; from 1 to 20 grams per liter of the iodate salt material and/or the chlorite salt material; and from 1 to 40 grams per liter of the phosphoric acid.
 12. The method according to claim 1, wherein the aqueous-based leaching solution is characterized by an oxidation reduction potential (ORP) in a range from 540 mV to 650 mV.
 13. An aqueous-based leaching solution for a precious metal, the aqueous-based leaching solution comprising: an iodide salt material; an iodate salt material, a chlorite salt material, or any combination thereof; and phosphoric acid.
 14. The aqueous-based leaching solution according to claim 13, wherein the iodide salt material is present in the solution at a concentration of at least 5 grams per liter but no greater than 90 grams per liter.
 15. The aqueous-based leaching solution according to claim 13, wherein the phosphoric acid is present in the solution at a concentration of at least 1 gram per liter but no greater than 40 grams per liter.
 16. The aqueous-based leaching solution according to claim 13, wherein the phosphoric acid is present in the solution at a concentration of at least 5 grams per liter but no greater than 20 grams per liter.
 17. The aqueous-based leaching solution according to claim 13, wherein: the iodide salt material comprises sodium iodide and/or potassium iodide; the iodate salt material comprises sodium iodate and/or potassium iodate; and the chlorite salt material comprises sodium chlorite and/or potassium chlorite.
 18. The aqueous-based leaching solution according to claim 13, wherein the solution is characterized by an oxidation reduction potential (ORP) in a range from 540 mV to 650 mV.
 19. The aqueous-based leaching solution according to claim 13, wherein the solution further comprises potassium bromide, sodium bromide, potassium chloride, sodium chloride, or any combination thereof.
 20. The aqueous-based leaching solution according to claim 13, wherein the solution is prepared by a process comprising: combining the iodide salt material, the phosphoric acid, the chlorite salt material and/or the iodate salt material, and water in an agitated tank. 